Added a C++ JSON library

See: https://github.com/nlohmann/json

doomsday/cmake/FindJson.cmake

@@ -0,0 +1,6 @@
+set (DE_JSON_DIR "${DE_EXTERNAL_SOURCE_DIR}/json")
+
+if (NOT TARGET json)
+    add_library (json INTERFACE)
+    target_include_directories (json INTERFACE "${DE_JSON_DIR}/include")
+endif ()

doomsday/external/json/include/nlohmann/adl_serializer.hpp

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <utility>
+
+#include <nlohmann/detail/conversions/from_json.hpp>
+#include <nlohmann/detail/conversions/to_json.hpp>
+
+namespace nlohmann
+{
+template<typename, typename>
+struct adl_serializer
+{
+    /*!
+    @brief convert a JSON value to any value type
+
+    This function is usually called by the `get()` function of the
+    @ref basic_json class (either explicit or via conversion operators).
+
+    @param[in] j         JSON value to read from
+    @param[in,out] val  value to write to
+    */
+    template<typename BasicJsonType, typename ValueType>
+    static void from_json(BasicJsonType&& j, ValueType& val) noexcept(
+        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
+    {
+        ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
+    }
+
+    /*!
+    @brief convert any value type to a JSON value
+
+    This function is usually called by the constructors of the @ref basic_json
+    class.
+
+    @param[in,out] j  JSON value to write to
+    @param[in] val     value to read from
+    */
+    template<typename BasicJsonType, typename ValueType>
+    static void to_json(BasicJsonType& j, ValueType&& val) noexcept(
+        noexcept(::nlohmann::to_json(j, std::forward<ValueType>(val))))
+    {
+        ::nlohmann::to_json(j, std::forward<ValueType>(val));
+    }
+};
+}